Fix capture set variable installation in Setup

Author: Linyxus

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -552,7 +552,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
       case tp: (TypeRef | AppliedType) =>
         val sym = tp.typeSymbol
         if sym.isClass then
-          sym == defn.AnyClass
+          sym == defn.AnyClass || !sym.isPureClass
             // we assume Any is a shorthand of {cap} Any, so if Any is an upper
             // bound, the type is taken to be impure.
         else
@@ -708,4 +708,4 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
   def postCheck()(using Context): Unit =
     for chk <- todoAtPostCheck do chk(ctx)
     todoAtPostCheck.clear()
-end Setup
+end Setup

tests/pos-custom-args/captures/cc-setup-impure-classes.scala

@@ -0,0 +1,5 @@
+import language.experimental.captureChecking
+
+trait Resource
+def id[X](x: X): x.type = x
+def foo[M <: Resource](r: M^): Unit = id(r)

tests/pos-custom-args/captures/future-traverse.scala

@@ -0,0 +1,16 @@
+import language.experimental.captureChecking
+
+trait Builder[-A, +C]
+trait BuildFrom[-From, -A, +C] {
+  def newBuilder(from: From): Builder[A, C]
+}
+
+trait Future[+T] { this: Future[T]^ =>
+  import Future.*
+  def foldLeft[R](r: R): R = r
+  def traverse[A, B, M[X] <: IterableOnce[X]](in: M[A]^, bf: BuildFrom[M[A]^, B, M[B]^]): Unit =
+    foldLeft(successful(bf.newBuilder(in)))
+}
+object Future {
+  def successful[T](result: T): Future[T] = ???
+}